If I were you I would try desperately to get a copy of the letter, even with the vendors info redacted if that makes them more comfortable.
If the AG office denies it (unlikely), we can probably file an FOIA request (is that correct, my legal scholars?), and they will wind up with more egg on their face.
I really think we should spin this as a hardcore push that the AGs office under Martha Coakley is abusing their authority to threaten and extort lawful businesses they don't "agree" with. It's ****ing bullshit, and the actions should be dragged out for all to see. The idiots will still laud her for doing it, but any of the educated democrats SHOULD have a problem with this... Just as I would have a huge issue with the AGs office threatening groups like Moms Demand Action.
ETA: If any of the AG aids are reading this (since we know we do have moles on this site). I'd just like to say **** you, you have no concept of service to your country. Go do something more worthwhile like harass the level IIIs that live all over this ****ing state instead of rot in jail. /rant
Mike